1. Field of Invention
The present invention relates to a water storage device for storing water used in a humidification system or the like and a fuel cell system including the water storage device. Particularly, the present invention relates to a technology for preventing damage attributable to volume expansion when water in the water storage device is frozen.
2. Description of Related Art
As countermeasures against global environmental problems, such as air pollution caused by automobile emissions and global warming caused by carbon dioxide, a fuel cell with clean emissions and high energy efficiency has been brought to attention. The fuel cell is an electrochemical device which converts the chemical energy of an electrochemical reaction between air and fuel gas, such as hydrogen gas or hydrogen-rich reformed gas, supplied to an electrolyte/electrode catalyst complex, directly into an electric energy. Among various types of the fuel cells, a solid polymer electrolyte fuel cell with a solid polymer electrolyte membrane is simply constructed, compact and low-cost, and has a high power density. Thus, the solid polymer electrolyte fuel cell is expected to be used as a power source for a movable body such as an automobile.
In the solid polymer electrolyte fuel cell, the solid polymer membrane has two functions, more specifically, a function as an ion conductive electrolyte and a function of separating air and fuel gas from each other within the cell. In order to realize the both functions, the solid polymer membrane is required to be water-saturated. Unless a water content of the solid polymer membrane is sufficient, an ionic resistance thereof is increased and air and fuel gas are mixed, affecting performance of power generation thereof.
A solid polymer membrane of a solid polymer electrolyte fuel cell in practical use tends to be dried at a hydrogen electrode side since protons produced at the hydrogen electrode are transported through the solid polymer membrane together with water molecules. Moreover, if the fuel gas or the air supplied to the fuel cell contains insufficient water vapor, the vicinity of the supply ports tends to be dried.
Therefore, in the solid polymer electrolyte fuel cell, it is required to humidify/moisten the solid polymer membrane, taking moisture from outside into the fuel cell. Usually, there is provided a humidification system in a fuel cell system for humidifying/moistening an electrolyte membrane and/or supplied fuel gas and air.
For a smooth start-up of the fuel cell system applied for an electric vehicle to be used in a cold region or the like, it is necessary to take measures against freezing of water in the humidification system. Japanese Patent Application publication Laid-Open No. 2000-149970 discloses a power generation system for an electric vehicle, in which coolant is used to heat up a water tank and melt frozen water therein at start-up of the system.